This invention relates to flexible membrane switches including a flexible, electrically insulative membrane with a series of parallel elongate conductive strips formed on one surface thereof, and an insulative substrate with parallel and elongate conductive strips formed on one of its surfaces. The membrane and substrate are mounted in spaced-apart relation with the strips of each facing the other and oriented perpendicular to the strips of the other. The membrane permits the closing of a selected circuit by application of finger or instrument pressure to a selected location on the membrane corresponding to the intersection of the selected membrane strip and substrate strip. When the pressure is released, the membrane returns to its normal, unstressed configuration and the circuit is open once again.
Flexible membrane switches have various applications including controls for household appliances, vending machines and telephones. One adaptation, particularly well suited for use in computer-based education and training, involves the use of substantially transparent materials for the substrate, membrane and the conductive strips. Viewing images on a CRT (cathode ray tube) screen through such a switch, a student is able to close selected circuits in response to instructions, test questions, a simulated flight pattern or other specific information.
Such a transparent membrane touch panel is shown in U.S. Pat. No. 4,085,302. Inventors Zenk, Johnson and Miller, two of whom are applicants herein, describe a spherically-contoured touch panel particularly well suited for use with a CRT screen. A glass substrate is provided with conductive strips of indium oxide, and the flexible polyester membrane is coated with conductive strips of gold. The membrane has a slightly shorter radius of curvature than does the substrate, and is thus self-supporting to maintain separation between the membrane strips and substrate strips so that all circuits are normally open. There are problems with this design--for example, the difficulty in molding the polyester membrane and its strips to the desired spherical contour and the obvious incompatibility with flat surfaces--which make a planar touch panel desirable. However, a flat polyester membrane is self-supporting only for panels of limited size, i.e., about four inches (10 cm.) square.
To provide support between the substrate and membrane, the above patent describes an electrically insulative grid approximately 0.0001 inch (0.0025 mm.) thick. This grid is elastically deformable to allow chosen opposing strips to touch one another responsive to finger pressure. This grid is expensive, shows wear after repeated cycles or deformation incidents, and reduces the transparency of the touch panel.
An alternative to the grid is a separator sheet shown in U.S. Pat. No. 3,617,666 to Brane granted Nov. 2, 1971. Two fabric layers 3 and 5 are held apart from one another by a separator sheet 4. Openings 12 are provided in the separator sheet. Sheet 4 is then positioned in the touch panel to align openings with the various strip intersections. Such a separator sheet detracts from the aesthetic appearance of the touch panel and reduces transparency. Further, the sharp edges at the openings create stress concentrations which, over repeated cycles, can damage both the separator sheet and the flexible membrane. Specifically, continued flexing of a membrane conductive strip across its associated edge eventually wears down the strip to increase its resistance or even permanently open the circuit. Continued flexing also gradually rounds the edges of the separator sheet. The debris from this process produces an insulating dust within the switch array causing a phenomenon known as "hard touch."
It is an object of this invention, therefore, to provide a simple, low-cost means for normally separating a touch panel flexible membrane from its opposed substrate. Another object is to provide such separation with minimal reduction in transparency of the touch panel, and to significantly increase the useful life of the panel in terms of its resistance to wear from repeated circuit connections.